Yevheniia Khyzhniak | Searcharxiv

arXiv2026

When positive and negative pairs differ in femtoscopy: residual Coulomb and isospin effects

We study charge-dependent modifications of identical-pion and identical-kaon femtoscopic correlation functions from two sources: the residual Coulomb field of the charged source and isospin-related hadronic dynamics. The residual Coulomb effect is modeled with a modified Retiere--Lisa blast-wave source, where the same emitted particles are propagated with positive and negative charge signs through an effective residual field. The residual Coulomb field produces a small but systematic positive-to-negative splitting of the correlation functions, strongest at low $k_T$ and sensitive to the effective charge, spatial width, and expansion velocity of the residual source. It also modifies the height and shape of the correlation function, complicating interpretation of the fitted radii. UrQMD 3.4 cascade calculations for Au+Au collisions at $\sqrt{s_{NN}}=7.7~\mathrm{GeV}$ show that charge-dependent splittings can also appear even without a residual Coulomb field. For pions, this difference is mainly driven by the initial isospin composition, while for kaons it is strongly affected by different $K^+$ and $K^-$ production mechanisms and subsequent hadronic evolution. These results show that residual Coulomb and isospin-related effects can compete, and neither can be interpreted reliably without constraining the other within the same model framework.

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arXiv2024

Pair momentum dependence of tilted source in heavy ion collisions

In non-central heavy-ion collisions, the particle-emitting source can be tilted away from the beam direction, an effect that becomes particularly significant at collision energies of a few GeV and lower. This phenomenon, manifest itself in many observables such as directed flow, polarization, and vorticity, is therefore important to investigate. In this paper, we study the consistency between the tilt extracted directly from the freeze-out distribution of pions and the tilt parameter obtained using the azimuthally sensitive femtoscopy (asHBT) method. Using the UrQMD model, we demonstrate a strong dependence of the tilt parameter extracted with asHBT on the momentum of the particle pair. Considering the experimental challenges in accessing low particle momenta - where the tilt parameter extracted with asHBT closely matches the tilt of the freeze-out distribution of pions - we propose an exponential extrapolation method to obtain the tilt of the entire freeze-out distribution. This approach aims to enhance the accuracy of experimental measurements of tilt in non-central heavy-ion collisions.

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